He might have already been trailing in the polls. There may have already been deep doubts among voters about whether he was too old for the job. But it took just one appalling, hard-to-watch stumble by US President Joe Biden last week to lead even the most sober of analysts to not just say he should withdraw from the presidential contest, but even question his capacity for office in the months remaining of his presidency.

That was despite Biden’s slip being made in a debate with a convicted felon and sexual abuser who was widely reported to have lied repeatedly on the night about the most basic of facts.